I expect that Siri will be very important, a step beyond one-word voice command.

The 8MP camera, with improved optics, sensor and electronics will be very nice to have. (And the 5MP camera in the iPhone 4 is very, very good.)

Here's an Apple page comparing iPhone models. It includes the tidbit that you can still order an iPhone 4 at a $100 discount. But note that it's an 8 GB iPhone 4, compared to the base model iPhone 4s, which has 16 GB. Considering the substantial improvement that lowercase s implies, that doesn't look like such a good deal. (I have a 16 GB Verizon iPhone 4, which was the base model when I bought it, and I certainly wouldn't want to go back to 8 GB.)

Apple today announced pre-orders of its iPhone 4S have topped one million in a single day, surpassing the previous single day pre-order record of 600,000 held by iPhone 4. ... The first day pre-orders for iPhone 4S have been the most for any new product that Apple has ever launched...

According to Pogue, Siri handles voice control while Dragon Dictation (unfilled) handles dictation.

[Added 10 13 11]

But the iPhone User Guide (iOS 5) doesn't say that:

On iPhone 4S, you can dictate text instead of typing it on the onscreen keyboard. For example, you can dictate a message in Mail or a note in Notes. Siri must be turned on in Settings > General > Siri.
Dictate text: From the onscreen keyboard, tap , then dictate. When you finish, tap Done. To add to the message, tap again.

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs